diff --git a/metagpt/memory/memory.py b/metagpt/memory/memory.py
index 2f4c9d20b..71d999049 100644
--- a/metagpt/memory/memory.py
+++ b/metagpt/memory/memory.py
@@ -73,7 +73,7 @@ class Memory:
             news.append(i)
         return news
 
-    def get_by_action(self, action) -> list[Message]:
+    def get_by_action(self, action: str) -> list[Message]:
         """Return all messages triggered by a specified Action"""
         return self.index[action]